Crystallographic data were recorded from the MoFe containing part of nitrogenase to 1.5 E resolution. Additional data sets were collected where the samples were pressurized with Xe, CO and NO, respectively. Preliminary indications suggested that subtle changes in the oxidation state of the Fe-S cluster may have contributed to a misassignment of the geometry. A data set was collected from a sample in which the oxidation state was controlled by reducing the crystal in mother liquor prior to flash-cooling. The resulting maps confirmed that the previous model was incorrect and a revised model is forthcoming.